Kids mature at different ages. Yes, she sounds unusual, but unless there are any underlying disabilities (mental or physical), just leave her be what she wants to be.
Our son stayed immature throughout his teen years. Didn't do anything with school friends outside school, throughout secondary, not even in the sixth form. Never went out with friends at the weekend. Basically his whole life was based on what we were doing. No girlfriends either. He'd never even gone to a pub or club by the time he left school. And certainly went nowhere unless we were with him.
He went to uni, but it was 2020 so it was all restricted due to covid, no face to face lectures, seminars, etc., so he basically never left his flat as all teaching and "socials" were online. He didn't really get friendly with his flat mates in the first year either, not helped by everyone being forced to study from home between Jan and April 2021! He opted out when they started talking about sharing a flat for year 2! He stayed on campus for year 2 in a new flat with new people and still didn't seem to be making friends, having a social life, etc.
We started to get more and more worried about him being reclusive and a "loner" as we'd hoped he'd come out of his shell at Uni. But he started getting friendly with his old year 1 flatmates mid way during year 2. Started going to their flat in town for the odd meal/drinks night. Still made no friends with anyone else. But then one of the flat mates announced they were leaving at end of year 2 and the others invited our son to take their place for the third year. It was an absolute revelation. He changed enormously in year 3. Massive growth in confidence. He started going out with them, making new friends (i.e. making friends with their wider friendship group, etc). By the end of year 3, he was a different person completely.
He went straight into a graduate scheme job the month after graduation. Huge firm, had to move to a different city. Again, we were worried about his maturity, making friends, settling into his first "proper" job, without us around (we were over 100 miles away). But he absolutely nailed it all. He was going out for after work drinks with work mates within the first week. Then he started wanting to follow his "home" football team at their away matches and just took it upon himself to get trains to matches all over the country (never been on a train before on his own!). Then decided he wanted a weekend in London to see a particular exhibition - organised it all himself, went on his own. Then he organised his own trip to the Silverstone Formula 1!
He "grew up" within less than a year, after being really immature for best part of 21 years and not really being a teenager at all.
So, OP, really don't worry. They mature at their own pace, as and when they want to. In our son's case, it was being away from home that gave him the nudge he needed to act like an adult as he knew it was all down to him as to whether he wanted to make friends, socialise, do things, or not, and he decided he wanted to and found it within himself to put himself out there!